Epitaxy is a technology for growth of thin crystal film. A variety of advanced electronic and optoelectronic devices are fabricated by using heteroepitaxial thin layer crystal structures. The quality of the epitaxial layers determines the device performance.
In general, the lattice constant and thermal expansion coefficient are the two important factors for the quality of epitaxial layers of materials. The strain induced by the difference in lattice constants between the epitaxial layer and the substrate creates misfit dislocations in the epitaxial layers when the film thickness reaches a certain thickness, namely the critical thickness. The misfit dislocations extend until they reach the edge of the wafer or the surface of the epitaxial layer during growth. The section of dislocation moving through the entire epilayer thickness is referred to as threading dislocation, and is detrimental to device performance.
Theories and some experimental results (e.g., M. Tachikawa, et al., Appl. Phys. Lett. 56, 2225, 1990 ) demonstrate that the formation and movement of a large number of dislocations in heterogeneous epitaxial materials often happens in the cooling stage after growth. This is true when the thermal expansion coefficient of the epitaxial layer is larger than that of the substrate, e.g., GaAs on Si. The large difference between the thermal expansion coefficients of the epitaxial layer and substrate (e.g., GaAs on Si ) produces a thermal stress strong enough to generate and move dislocations. The thermal stress induced dislocations and concurrent dislocation propagation to the epitaxial layer surface is the main mechanism of dislocation generation in the cooling stage. It should be noted that in the GaAs on Si case, the GaAs epitaxial layer is always in tensile stress during cooling. Based on the mechanical properties of thin films and experimental results, the film in tension always causes plastic deformation problems, besides microscopic dislocations, such as macroscopic cracking and dehesion (or debonding). The film in compression can also create dislocations, but the movement of dislocations to the surface of epitaxial layers is suppressed by compressive thermal stress, which does not cause the cracking problem. This discussion is also applicable to heterogeneous material bonding. Since the role of the lattice constant is understood, we concentrate on the role of thermal stress here.
High quality integration of heterogeneous materials with large differences in thermal expansion coefficients and lattice constants, such as GaAs on Si, by means of wafer bonding technologies is a serious problem. Many research groups all over the world are pursuing this target. To realize the target we tried several different technologies, including:
1) direct bonding,
2) thin intermediate metal film bonding, and
3) RT (room temperature) bonding.
Direct bonding is conducted under high temperature and high pressure. Our experimental results indicate that the bonded wafers have very strong bonding strength but the wafers often crack or fracture into small pieces due to strong thermal stress. Obviously we cannot create a compliant composite substrate by using this technology.
Bonding by thin intermediate metal film can be implemented at high temperatures and pressures. We chose a metal, such as Cr, with a high melting point and good adhesion to semiconductors as the intermediate material. The crack and fracture problem seems to be alleviated to some extent, but the bonding strength is not strong enough to withstand the thermal stress generated. Debonding often occurred.
RT bonding is a new technology for heterogeneous material integration at room or low temperature and under low pressure. We achieved a bonding interface thermally stable up to 700.degree. C. Even though this technology works well, a substrate bonded by this method is still unable to function as a compliant substrate and thermal stress self-adjuster during growth.